Urea crystals, best grown from alcoholic solution, have a long primatic needle-like formation (see FIG. 1), which is caused by rapid crystal growth on the 111 crystal face. Unfortunately for laser use, it is desirable that crystals having significant dimensions on the 110 and 110 faces be available.
Certain materials including ammonium bromide have been indicated as solution additives which retard growth at the 111 face of a urea crystal. Nevertheless, relatively rapid, consistent production of single urea crystals having substantial 110 and 110 dimensions has not been demonstrated in the art.
Bunn, Proc. Roy. Soc. London, A-141 (1933) 567-593 describes the use of solution additives to modify crystal habit and particularly teaches that NH.sub.4 Cl and NH.sub.4 Br (the latter more strongly) cause the formation of basal planes which reduce or obliterate the 111 faces of urea crystals formed from alcoholic solutions (see FIG. 2).
Gaedeke et al., Krist U. Tech., 14 (1979) 913-922; and Krist. U. Tech., 15 (1980) 557-564 describe the effect of various additives on the crystal habit and transportation and storage properties of urea.
Lee et al., J. of Crystal Growth, 32 (1976) 363-370 report a study of the nucleation and growth of urea from water-alcohol solvent mixtures.
Kurtz et al., J. of Appl. Phys., 39 (1968) 3798-3813; Betzler et al., J. of Mol. Structure, 47 (1978) 393-396; and Bauerle et al., Phys. Stat. Sol., (a) 42 (1977) K-119 report second harmonic characteristics of urea crystals.